This invention relates generally to acoustic touch systems, and more particularly, to processing signals received from acoustic touch pads and other touch surfaces.
Touch events are signified by an acoustic interaction between a surface of a touch pad or other touch surface and an object such as a fingertip or stylus. A plurality of transducers may be provided to acquire the acoustic signals at different locations within the touch pad. Signals representing two channels of the acoustic signals are output via a cable to an analog to digital (A/D) converter. These digital signals are processed to extract useable touch location information.
Before use, the touch pad may be calibrated and/or the calibration data stored which defines both x and y coordinates of a specific location on the touch pad surface. Signal information from live touch data then may be compared to and matched appropriately with calibration data to identify the location of the touch event in two dimensions across the surface.
Due to different types of touch events, a broad voltage range is experienced at the input to the A/D converter which has an operating range within which it converts data. If the operating range is set to accommodate the highest maximum voltage level, voltages at the low end of the scale may not register and/or suffer significantly from quantization error, and, as a result, corresponding touch events may not be recognized. Also, when the input to the A/D converter is higher than the maximum level of the operating range, the output of the A/D converter is “railed”, frequency domain analysis produces a “splatter” across the spectrum, and the touch event may not be recognized or may even be incorrectly interpreted.
Therefore, a need exists for methods and apparatus to process acoustic signals from an acoustic touch pad or other touch surface to enable a more robust detection of touch events and reduce quantization error. Certain embodiments of the present invention are intended to meet these needs and other objectives that will become apparent from the description and drawings set forth below.